Learn Russian: Он стрелял в воздух, а не в людей.

In Russian, to express shooting 'at' a target, you use в plus the accusative case, treating it like a direction of motion (shooting 'into'). This is why the sentence uses the accusative forms в воздух and в людей.

Why is the word for 'people' in the form людей?

Because the target of the shooting requires the accusative case, we need the accusative plural of the word for people. For animate nouns (living beings), the accusative plural is identical to the genitive plural, which is людей.

Could I use the perfective verb выстрелил instead of the imperfective стрелял?

You could, but the meaning would slightly change. The perfective выстрелил would mean he fired one single, completed shot. The imperfective стрелял focuses on the ongoing action itself or implies he fired multiple times.

Why is the conjunction а used in а не instead of но?

The phrase а не is used to create a direct contrast between two mutually exclusive things, translating to 'and not' or 'rather than'. While но also means 'but', it introduces an unexpected clause or limitation, whereas а simply opposes the two targets directly.